1. Field of the Invention
This invention relates to an electret sheet which is suitable for use as an air filter, a mask, or the like, and to a method of manufacturing the same. The air filter can be used for purifying the air contained within a clean room, air which is to be supplied to buildings, or gas discharged by factories. The air filter can be attached to an air cleaner, a vacuum cleaner, a copying machine, or the like.
2. Description of the Prior Art
There have been proposed techniques for efficiently collecting dust in the air by use of an electrified sheet. Japanese Patent Publication No. 56-47299 discloses a method of manufacturing an electret sheet as follows: A film made of a dielectric material is electrified and then split into fibers, which are formed into the electret sheet.
With the method mentioned above, an electrified film having a great amount of electrostatic charge can be obtained, but it is impossible to form fibers with a sufficiently small width from the film. Accordingly, the cross-section of each fiber is rectangular, which results in a low collection efficiency of the electret sheet formed from the fibers. When a filter formed from the electret sheet is used over a long period of time, the collection efficiency and the amount of electrostatic charges of the filter gradually decrease with the passage of time, based on the neutralization of static electricity, and will not be restored.
There is disclosed a technique in Japanese Patent Publication No. 53-40073 as follows: A film made of an insulating film and a sheet formed from fibers are successively disposed in that order on an earthed electrode, and then a sheet formed from the fibers is electrified. The resultant electret sheet has a small amount of electrostatic charge, and thus a low collection efficiency.